In a torque converter comprising a lock-up clutch, control of a differential pressure at the front and rear of the lock-up clutch (lock-up differential pressure) locks/disengages the lock-up clutch. The lock-up differential pressure is gradually raised from a predetermined initial differential pressure in order to shift the torque converter from a converter state to a lock-up state. The torque converter is shifted from the converter state to the lock-up state through a slip state. The lock-up clutch is released or disengaged in the converter state, slips in the slip state, and is locked in the lock-up state.
Japanese Patent Application Laid-Open No. 2002-205576 published by the Japanese Patent Office in 2002 discloses control of a lock-up clutch which improves fuel economy by performing locking up from a low vehicle speed. According to the abovementioned conventional technology, when the vehicle speed does not increase on an ascending slope or the like, a transmission ratio or gear ratio of the automatic transmission is changed to Low, while maintaining the lock-up state to amplify the transmission torque of the automatic transmission.